The present invention relates to an improved heat exchanger. The heat exchanger of the present invention is advantageous for use in a carbon black production process.
Carbon blacks are generally produced in a furnace-type reactor by pyrolyzing a hydrocarbon feedstock with hot combustion gases to produce combustion products containing particulate carbon black. A variety of methods for producing carbon blacks are generally known.
In one type of a furnace carbon black reactor, such as shown in U.S. Pat. No. 3,401,020 to Kester et al., or U.S. Pat. No. 2,785,964 to Pollock, hereinafter xe2x80x9cKesterxe2x80x9d and xe2x80x9cPollockxe2x80x9d respectively, a fuel, preferably hydrocarbonaceous, and an oxidant, preferably air, are injected into a first zone and react to form hot combustion gases. A hydrocarbon feedstock in either gaseous, vapor or liquid form is also injected into the first zone whereupon pyrolysis of the hydrocarbon feedstock commences. In this instance, pyrolysis refers to the thermal decomposition of a hydrocarbon. The resulting combustion gas mixture, in which pyrolysis is occurring, then passes into a reaction zone where completion of the carbon black forming reaction occurs.
In another type of a furnace carbon black reactor a liquid or gaseous fuel is reacted with an oxidant, preferably air, in the first zone to form hot combustion gases. These hot combustion gases pass from the first zone, downstream through the reactor, into a reaction zone and beyond. To produce carbon blacks, a hydrocarbonaceous feedstock is injected at one or more points into the path of the hot combustion gas stream. The hydrocarbonaceous feedstock may be liquid, gas or vapor, and may be the same or different than the fuel utilized to form the combustion gas stream. Generally the hydrocarbonaceous feedstock is a hydrocarbon oil or natural gas, however other hydrocarbonaceous feedstocks such as acetylene are known in the art. The first (or combustion) zone and the reaction zone may be divided by a choke or zone of restricted diameter which is smaller in cross section than the combustion zone or the reaction zone. The feedstock may be injected into the path of the hot combustion gases upstream of, downstream of, and/or in the restricted diameter zone. Furnace carbon black reactors of this type are generally described in U.S. Patent Reissue No. 28,974 and U.S. Pat. No. 3,922,335 the disclosure of each being incorporated herein by reference.
In generally known reactors and processes, the hot combustion gases are at a temperature sufficient to effect pyrolysis of the hydrocarbonaceous feedstock injected into the combustion gas stream. In one type of reactor, such as disclosed in Kester, feedstock is injected, at one or more points, into the same zone where combustion gases are being formed. In other type reactors or processes the injection of the feedstock occurs, at one or more points, after the combustion gas stream has been formed. In either type of reactor, since the hot combustion gas stream is continuously flowing downstream through the reactor, pyrolysis continuously occurs as the mixture of feedstock and combustion gases passes through the reaction zone. After carbon blacks having the desired properties are formed, the temperature of the effluent is lowered to a temperature such that pyrolysis is stopped, thereby halting the further production of carbon blacks.
After pyrolysis is stopped, the carbon black containing stream generally passes through a heat exchanger to further cool the mixture.
A disadvantage with many heat exchangers utilized in carbon black production processes is that the combustion air heat exchangers operate in a fouled condition. Fouling occurs through a build-up of carbon black and other deposits on the heat exchange surface, in particular the internal heat exchange surface which comes in contact with the carbon black containing stream.
The fouling of heat exchanger surfaces in a carbon black processes is often cyclical in nature. A period of gradual fouling can be followed by a faster defouling, followed by another period of fouling etc. The fouling of heat exchanger surfaces in a carbon black production process can lead to a number of problems including:
less effective heat transfer resulting in lower air exiting temperatures (lower air preheat) and thus lower carbon black yields and production rates;
less effective heat transfer necessitating higher carbon black stream entering temperatures in order to achieve a desired air exiting temperature, thus increasing the stresses imposed on the heat exchanger materials;
less effective heat transfer resulting in variations in air exiting temperatures which result in variations in carbon black morphology;
an increase in heat exchanger pressure drop, which can result in lower production rates and greater stresses in the heat exchanger materials;
possible equipment damage;
and the tendency of the deposits to harden over time while on the heat exchange surface resulting in the possibility that the hardened deposits will re-enter the carbon black stream contaminating the carbon black product.
We have discovered that the fouling of a heat exchange surface in a heat exchanger may be reduced or minimized by maintaining a temperature difference between the heat exchange surface and a fluid stream in contact with the heat exchange surface. For example, fouling of the heat exchange surfaces in a combustion air heat exchanger may be reduced or minimized by maintaining the temperature difference between the heat exchange surface and the carbon black stream in contact with the heat exchange surface such that the temperature difference is sufficient to inhibit fouling of the heat exchange surface. In the case of a combustion air heat exchanger, the temperature difference between the heat exchange surface and the carbon black stream is preferably not greater than 500xc2x0 F., more preferably not greater than 300xc2x0 F. and even more preferably not greater than 100xc2x0 F. Preferably the temperature difference is maintained throughout the heat exchanger. Depending on the carbon black production process, it is possible that at a particular temperature difference, fouling of the heat exchange surface may be substantially eliminated.
According to the present invention, a method for reducing the fouling of a heat exchange surface of a heat exchanger for transferring heat between a first fluid stream and a second fluid stream each passing through the heat exchanger and in communication with the heat exchange surface, comprises maintaining the temperature difference between the heat exchange surface and a fluid stream so that the temperature difference is sufficient to inhibit fouling of the heat exchange surface. The temperature difference may be maintained by controlling the velocity of one of the fluid streams. With reference to a combustion air heat exchanger of the type utilized in a carbon black production process, the present invention provides a method for reducing the fouling of a heat exchange surface of a heat exchanger for tranferring heat between a carbon black stream and an air stream each passing through the heat exchanger and in communication with the heat exchange surface, the method comprising: maintaining the temperature difference between the heat exchange surface and the carbon black stream so that the temperature difference is sufficient to inhibit fouling of the heat exchange surface. The temperature difference may be maintained by controlling the velocity of the air stream. As used in the foregoing description, xe2x80x9ccommunicationxe2x80x9d refers to the ability to transfer heat between the stream and the heat exchange surface.
In accordance with another aspect of the present invention, a heat exchanger for transferring heat between a first and second fluid comprises:
a heat exchange surface;
means for passing the first fluid stream past the heat exchange surface;
means for passing the second fluid stream past the heat exchange surface; wherein the means for passing the first fluid stream past the heat exchange surface enable a temperature difference to be maintained between the heat exchange surface and the second fluid stream sufficient to inhibit fouling of the heat exchange surface. Preferably the temperature difference maintained between the heat exchange surface and the first fluid is not greater than 500xc2x0 F., more preferably not greater than 300xc2x0 F. and even more preferably not greater than 100xc2x0 F. Preferably the temperature difference is maintained throughout the entire heat exchanger. The passing of the first fluid and second fluid past the heat exchange surface occurs sufficiently proximate to the heat exchange surface to allow heat transfer between each fluid and the heat exchange surface. For example each fluid stream may be in contact with the heat exchange surface.
With reference to a carbon black production process, a heat exchanger for transferring heat between a carbon black stream generated by a carbon black production process and an air stream comprises;
a heat exchange surface;
means for passing the carbon black stream past the heat exchange surface;
means for passing the air stream past the heat exchange surface; wherein the means for passing the air stream past the heat exchange surface enable a temperature difference to be maintained between the heat exchange surface and and the carbon black stream of not greater than 500xc2x0 F., more preferably not greater than 300xc2x0 F. and even more preferably not greater than 100xc2x0 F. Preferably the temperature difference is maintained throughout the entire heat exchanger. The passing of the carbon black stream and the passing of the air stream past the heat exchange surface occur sufficiently proximate to the heat exchange surface to allow heat transfer between each stream and the heat exchange surface. For example, the carbon black stream and the air stream may each be in contact with the heat exchange surface.
In accordance with another aspect of the present invention there is provided a process for producing carbon blacks comprising:
forming a stream of hot combustion gases by reacting a preheated air stream with a fuel;
passing the stream of hot combustion gases through a reactor;
injecting feedstock into the stream of hot combustion gases at one or more points to form an effluent and start pyrolysis of the feedstock in the effluent;
cooling the effluent after the formation of carbon blacks to stop pyrolysis and obtain a resultant carbon black stream comprising combustion gases and carbon black;
passing the carbon black stream past a heat exchange surface to cool the carbon black stream;
passing an air stream past the heat exchange surface to create the preheated air stream;
further cooling the carbon black stream in a secondary cooler to a temperature that prevents damage to the means utilized for separating and collecting the carbon black product; and separating and collecting carbon black product, wherein the temperature difference between the heat exchange surface and the carbon black stream is not greater than 500xc2x0 F., more preferably not greater than 300xc2x0 F. and even more preferably not greater than 100xc2x0 F.
An advantage of the method and the heat exchanger of the present invention is that the heat exchange surface is less prone to fouling than conventional heat exchangers.
An advantage of the combustion air heat exchanger of the present invention is that the combustion air heat exchanger resists fouling of the heat exchange surface thereby minimizing the occurrence of the problems described above which beset conventional combustion air heat exchangers.
An advantage of the process for producing carbon blacks of the present invention is that the process produces a high purity carbon black product with substantially consistent morphology.
Further details and advantages of the features of the present invention are set forth in the following more detailed description.